Once and for all (?) : Previous to 1492, discoveries of new lands were deeply hidden trade/national secrets. The official version of history claims there' no "proof" of this. But it isn't looking very hard for it, either.

Case in point : the ear of corn (maize) carved into Roslyn Chapel in Scotland.

Now there's another one (above) : a sketch of a kangaroo in a book compiled previous to the supposed "discovery" of Australia by Europeans.

It is very likely that the Portuguese visited Australia before the Dutch, however the Lisbon earthquake and tsunami of 1755 destroyed the Portguese Royal Archives. Along with written evidence which would prove that the Portuguese visited Australia before the Dutch.